1. Field of the Invention
This invention relates to an image motion detector system and, more particularly, to an improved two-axis system utilizing a single image detector.
2. Description of the Prior Art
Image motion detectors have great utility for use in aircraft movement with respect to a frame of reference such as the three-axis inertial space through which the aircraft moves. An aircraft can generally be held at a fixed altitude (h), thereby requiring detailed information only with respect to the other two axes (X, Y). Such information is necessary for orienting and stabilizing sensitive and accurate equipment located on the aircraft, such as cameras or telescopes, or for use in connection with navigation systems for determining, for example, the angular rate of the aircraft relative to a point on the earth. This information is necessary to determine the effect of the Doppler shift as the aircraft passes over a given reference point on the earth. This Doppler shift is proportional to the ratio of velocity over altitude (V/H or V/R), which is employed with Doppler navigation techniques.
In the prior art, single-axis systems have included rotating discs of the episcotister type having a plurality of opaque and transparent grid bands emanating radially from the center of the disc. An image is focused on a portion of the disc, corresponding to one of the axes about which information is sought. A reference signal, for example, a light-emitting diode, is also focused on the disc in the same plane as the image. Separate photo detectors, that is, one corresponding to the image and one corresponding to the reference source, respond to the image.
Information with respect to a second axis is obtainable by focusing the same image on the disc on a radial that is displaced by 90.degree. with respect to the first image. Another reference light-emitting diode, located in the same plane as the second axis, is used. A photo diode detects the image passing through the disc in the second axis and compares the output with still another photo diode receiving the second axis reference signal. Appropriate electronics determine a second signal, indicative of the motion of the image in the second axis.
Such prior art two-axis systems therefore require a pair of photo diodes located on orthogonal radii, for detecting the images from the source, and a first and second reference photo diode located in the same plane as said image photo diodes. Systems of this type have been built and constructed and have operated satisfactorily. One such system has included the apparatus described and claimed in the U.S. Pat. No. 3,500,050 to M. J. Hillman, issued Mar. 10, 1970, for "Image Motion Detector and Stabilizer."
These prior art two-axis systems have used beam splitters located in the optical path to ensure that the light image directed to the "X" aperture was the same as the light image directed to the "Y" aperture. This approach was the result of an attempt to increase the accuracy of the system. Each sensor therefore picks up motion in directions that are 90.degree. with respect to the other sensor. In the same manner, a single reference light source could be used with a beam splitter and a pair of orthogonally located photo diodes.
Unfortunately, optical beam splitters used in the present day two-axis systems have created problems arising from their great weight and bulk. Alternatives have included the use of a pair of rotating discs. However, the added complications caused by the second disc and its drive mechanism and the timing relationships between the two discs have more than offset any advantages to be gained over the use of a beam splitter with a single disc.